Like virtually all modern operating systems, Embedded Xinu supports preemptive multitasking, which makes it appear that multiple threads are executing at the same time on the same processor. Support for preemptive multitasking consists of support for multiple threads combined with a preemption mechanism.
Embedded Xinu supports multiple threads, but only one can execute at a time. A thread context refers to the saved state of a thread, primarily CPU registers. Embedded Xinu platforms must implement two functions to allow creating new threads and switching between threads using their thread contexts:
arch_setup_stack(), which is responsible for setting up the stack of a new thread to include an initial thread context and procedure arguments. This routine is typically implemented in C. It is called internally by
ctxsw(), which is responsible for switching between threads. More specifically, this routine must save the thread context of the current thread and restore the thread context of the new thread. This routine is always implemented in assembly language.
Since different CPU architectures use different registers and calling conventions,
the size and format of a thread context varies depending on the CPU architecture.
Note that since thread contexts are created in both
ctxsw(), these two routines must create contexts that are compatible, at least to the extent that
ctxsw() can either start a new thread or resume an existing thread.
See Preemptive multitasking (ARM) for an example.
Preemption occurs when the timer interrupt occurs and Embedded Xinu attempts to reschedule the currently executing thread, which results in a call to
ctxsw(), described above, that may switch to a different thread context. (We say may because the code is written such that
ctxsw() is called when the same thread is rescheduled to itself, in which case
ctxsw() restores the saved context immediately and is a no-op.) The means of generating a timer interrupt is platform-dependent and may even differ among platforms that share the same CPU architecture. For an example, see BCM2835 System Timer, which is used in the Raspberry Pi, an ARM-based platform.